Target for extreme ultraviolet light source

ABSTRACT

Techniques for forming a target and for producing extreme ultraviolet light include releasing an initial target material toward a target location, the target material including a material that emits extreme ultraviolet (EUV) light when converted to plasma; directing a first amplified light beam toward the initial target material, the first amplified light beam having an energy sufficient to form a collection of pieces of target material from the initial target material, each of the pieces being smaller than the initial target material and being spatially distributed throughout a hemisphere shaped volume; and directing a second amplified light beam toward the collection of pieces to convert the pieces of target material to plasma that emits EUV light.

TECHNICAL FIELD

The disclosed subject matter relates to a target for an extremeultraviolet (EUV) light source.

BACKGROUND

Extreme ultraviolet (EUV) light, for example, electromagnetic radiationhaving wavelengths of around 50 nm or less (also sometimes referred toas soft x-rays), and including light at a wavelength of about 13 nm, canbe used in photolithography processes to produce extremely smallfeatures in substrates, for example, silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material that has an element, for example, xenon,lithium, or tin, with an emission line in the EUV range into a plasmastate. In one such method, often termed laser produced plasma (LPP), theplasma can be produced by irradiating a target material, for example, inthe form of a droplet, plate, tape, stream, or cluster of material, withan amplified light beam that can be referred to as a drive laser. Forthis process, the plasma is typically produced in a sealed vessel, forexample, a vacuum chamber, and monitored using various types ofmetrology equipment.

SUMMARY

In one general aspect, a method includes releasing an initial targetmaterial toward a target location, the target material including amaterial that emits extreme ultraviolet (EUV) light when converted toplasma; directing a first amplified light beam toward the initial targetmaterial, the first amplified light beam having an energy sufficient toform a collection of pieces of target material from the initial targetmaterial, each of the pieces being smaller than the initial targetmaterial and being spatially distributed throughout a hemisphere shapedvolume; and directing a second amplified light beam toward thecollection of pieces to convert the pieces of target material to plasmathat emits EUV light.

Implementations can include one or more of the following features.

The EUV light can be emitted from the hemisphere shaped volume in alldirections.

The EUV light can be emitted from the hemisphere shaped volumeisotropically. The initial target material can include a metal, and thecollection of pieces can include pieces of the metal. The metal can betin.

The hemisphere shaped volume can define a longitudinal axis along adirection that is parallel to a direction of propagation of the secondamplified light beam and a transverse axis along a direction that istransverse to the direction of propagation of the second amplified lightbeam, and directing the second amplified light beam toward thecollection of pieces can include penetrating into the hemisphere shapedvolume along the longitudinal axis. The majority of the pieces in thecollection of pieces can be converted to plasma. The first amplifiedlight beam can be a pulse of light having a duration of 150 ps and awavelength of 1 μm.

The first amplified light beam can be a pulse of light having a durationof less than 150 ps and a wavelength of 1 μm.

The first amplified light beam can include two pulses of light that aretemporally separated from each other. The two pulses can include a firstpulse of light and a second pulse of light, the first pulse of lighthaving a duration of 1 ns to 10 ns, and the second pulse of light havinga duration of less than 1 ns.

The first and second amplified light beams can be beams of pulses.

The first amplified light beam can have an energy that is insufficientto convert the target material to plasma, and the second amplified lightbeam have an energy that is sufficient to convert the target material toplasma.

A density of the pieces of target material can increase along adirection that is parallel to a direction of propagation of the secondamplified light beam.

The pieces of target material in the hemisphere shaped volume can have adiameter of 1-10 μm.

In another general aspect, a target system for an extreme ultraviolet(EUV) light source includes pieces of a target material distributedthroughout a hemisphere shaped volume, the target material including amaterial that emits EUV light when converted to plasma; and a planesurface adjacent to the hemisphere shaped volume and defining a frontboundary of the hemisphere shaped volume, the front boundary beingpositioned to face a source of an amplified light beam. The hemisphereshaped volume faces away from the source of the amplified light beam.

Implementations can include one or more of the following features. Thehemisphere shaped volume can have a cross-sectional diameter in adirection that is transverse to a direction of propagation of theamplified light beam, and a maximum of the cross-sectional diameter canbe at the plane surface.

A density of the pieces of the target material in the hemisphere shapedvolume can increase along a direction that is parallel to a direction ofpropagation of the amplified light beam.

At least some of the pieces can be individual pieces that are physicallyseparated from each other.

The hemisphere shaped volume can be irradiated with an amplified lightbeam having sufficient energy to convert the individual pieces of thetarget material to plasma, and the hemisphere shaped target can emit EUVlight in all directions.

The target material droplet can be part of a stream of target materialdroplets that are released from a nozzle, and the target system also caninclude a second target material droplet that is separate from thetarget material droplet and released from the nozzle after the targetmaterial droplet. The target system also can include the nozzle.

The source of the amplified light beam can be an opening in a chamberthat receives the target material droplet.

In another general aspect, an extreme ultraviolet (EUV) light sourceincludes a first source that produces a pulse of light; a second sourcethat produces an amplified light beam; a target material deliverysystem; a chamber coupled to the target material delivery system; and asteering system that steers the amplified light beam toward a targetlocation in the chamber that receives a target material droplet from thetarget material delivery system, the target material droplet including amaterial that emits EUV light after being converted to plasma. Thetarget material droplet forms a target when struck by the pulse oflight, the target including a hemisphere shaped volume having pieces ofthe target material throughout the volume, and a plane surfacepositioned between the hemisphere shaped volume and the second source.

Implementations can include the following feature. The pulse of lightcan be 150 ps or less in duration.

Implementations of any of the techniques described above may include amethod, a process, a target, an assembly for generating a hemisphereshaped target, a device for generating a hemisphere shaped target, a kitor pre-assembled system for retrofitting an existing EUV light source,or an apparatus. The details of one or more implementations are setforth in the accompanying drawings and the description below. Otherfeatures will be apparent from the description and drawings, and fromthe claims.

DRAWING DESCRIPTION

FIG. 1A is a perspective view of an exemplary hemisphere shaped targetfor an EUV light source.

FIG. 1B is a side view of the exemplary hemisphere shaped target of FIG.1A.

FIG. 1C is a front cross-sectional view of the exemplary hemisphereshaped target of FIG. 1A along the line 1C-1C.

FIG. 1D is a plot of an exemplary density as a function of locationwithin the hemisphere shaped target of FIG. 1A.

FIG. 2A is a block diagram of an exemplary laser produced plasma extremeultraviolet light source.

FIG. 2B is a block diagram of an example of a drive laser system thatcan be used in the light source of FIG. 2A.

FIG. 3A is a top plan view of another laser produced plasma extremeultraviolet (EUV) light source and a lithography tool coupled to the EUVlight source.

FIGS. 3B and 3C are top views of a vacuum chamber of the EUV lightsource of FIG. 3A at two different times.

FIG. 3D is a partial side perspective view of the EUV light source ofFIG. 3A.

FIG. 3E is a cross-sectional plan view of the EUV light source of FIG.3D taken along the line 3E-3E.

FIG. 4 is a flow chart of an exemplary process for forming a hemisphereshaped target.

FIG. 5 is a plot of an exemplary waveform for transforming a targetmaterial droplet into a hemisphere shaped target.

FIGS. 6A-6D are side views of a target material droplet transforminginto a hemisphere shaped target through interactions with the waveformof FIG. 5.

FIGS. 7A and 7B are plots of exemplary density profiles as a function ofspatial location.

FIGS. 8A and 8B are plots of the target size, which shows the spatialextent of a hemisphere shaped target, as a function of time.

FIG. 9 is a plot of another exemplary waveform for transforming a targetmaterial droplet into a hemisphere shaped target.

FIGS. 10A-10E are side views of a target material droplet transforminginto a hemisphere shaped target through interactions with the waveformof FIG. 9.

FIGS. 11A-11C are plots of exemplary density profiles as a function ofspatial location.

DESCRIPTION

Referring to FIG. 1A, a perspective view of an exemplary target 5 isshown. The hemisphere shape and gently sloped density profile of thetarget 5 enables the target 5 to provide additional EUV light, increasedconversion efficiency, and EUV light that is radially emitted outwardfrom the target in all directions. The hemisphere shape can be a half ofa sphere or any other portion of a sphere. However, the hemisphere shapecan take other forms. For example, the hemisphere shape can be a partialoblate or prolate spheroid.

The target 5 can be used in a laser produced plasma (LPP) extremeultraviolet (EUV) light source. The target 5 includes a target materialthat emits EUV light when in a plasma state.

The target material can be a target mixture that includes a targetsubstance and impurities such as non-target particles. The targetsubstance is the substance that is converted to a plasma state that hasan emission line in the EUV range. The target substance can be, forexample, a droplet of liquid or molten metal, a portion of a liquidstream, solid particles or clusters, solid particles contained withinliquid droplets, a foam of target material, or solid particles containedwithin a portion of a liquid stream. The target substance, can be, forexample, water, tin, lithium, xenon, or any material that, whenconverted to a plasma state, has an emission line in the EUV range. Forexample, the target substance can be the element tin, which can be usedas pure tin (Sn); as a tin compound, for example, SnBr₄, SnBr₂, SnH₄; asa tin alloy, for example, tin-gallium alloys, tin-indium alloys,tin-indium-gallium alloys, or any combination of these alloys. Moreover,in the situation in which there are no impurities, the target materialincludes only the target substance. The discussion below providesexamples in which the target material is a target material droplet madeof molten metal. In these examples, the target material is referred toas the target material droplet. However, the target material can takeother forms.

Irradiating the target material with an amplified light beam ofsufficient energy (a “main pulse” or a “main beam”) converts the targetmaterial to plasma, thereby causing the target 5 to emit EUV light. FIG.1B is a side view of the target 5. FIG. 1C is a front cross-sectionalview of the target 5 along the line 1C-1C of FIG. 1A.

The target 5 is a collection of pieces of target material 20 distributedin a hemisphere shaped volume 10. The target 5 is formed by striking atarget material with one or more pulses of radiation (a “pre-pulse”)that precede (in time) the main pulse to transform the target materialinto a collection of pieces of target material. The pre-pulse isincident on a surface of the target material and the interaction betweenthe initial leading edge of the pre-pulse and the target material canproduce a plasma (that does not necessarily emit EUV light) at thesurface of the target material. The pre-pulse continues to be incidenton the created plasma and is absorbed by the plasma over a period thatis similar to the temporal duration of the pre-pulse, about 150picoseconds (ps). The created plasma expands as time passes. Aninteraction between the expanding plasma and the remaining portion ofthe target material can generate a shock wave that can act on the targetmaterial non-uniformly, with the center of the target material receivingthe brunt of the shock wave. The shock wave can cause the center part ofthe target material to break into particles that expand in threedimensions. However, because the center part also experiences force inan opposite direction from the expanding plasma, a hemisphere ofparticles can be formed instead of a sphere.

The pieces of target material 20 in the collection can be non-ionizedpieces or segments of target material. That is, the pieces of targetmaterial 20 are not in a plasma state when the main pulse strikes thetarget 5. The pieces or segments of target material 20 can be, forexample, a mist of nano- or micro-particles, separate pieces or segmentsof molten metal, or a cloud of atomic vapor. The pieces of targetmaterial 20 are bits of material that are distributed in a hemisphereshaped volume, but the pieces of target material 20 are not formed as asingle piece that fills the hemisphere shaped volume. There can be voidsbetween the pieces of target material 20. The pieces of target material20 can also include non-target material, such as impurities, that arenot converted to EUV light emitting plasma. The pieces of targetmaterial 20 are referred to as the particles 20. Individual particles 20can be 1-10 μm in diameter. The particles 20 can be separated from eachother. Some or all of the particles 20 can have physical contact withanother particle.

The hemisphere shaped volume 10 has a plane surface 12 that defines afront boundary of the hemisphere shaped volume 10, and a hemisphereshaped portion 14 that extends away from the plane surface in adirection “z.” When used in a EUV light source, a normal 15 of the planesurface 12 faces an oncoming amplified light beam 18 that propagates inthe “z” direction. The plane surface 12 can be transverse to directionof propagation of the oncoming amplified light beam 18, as shown inFIGS. 1A and 1B, or the plane surface 12 can be angled relative to theoncoming beam 18.

Referring also to FIG. 1D, the particles 20 are distributed in thehemisphere shaped volume 10 with an exemplary density gradient 25 thathas a minimum at the plane surface 12 of the target 5. The densitygradient 25 is a measure of the density of particles in a unit volume asa function of position within the hemisphere shaped volume 10. Thedensity gradient 25 increases within the target 5 in the direction ofpropagation (“z”) of the main pulse, and the maximum density is on aside of the target 5 opposite from the side of the plane surface 12. Theplacement of the minimum density at the plane surface 12 and the gradualincrease in the density of the particles 20 results in more of the mainpulse being absorbed by the target 5, thereby producing more EUV lightand providing a higher conversion efficiency (CE) for a light sourcethat uses the target 5. In effect, this means that enough energy isprovided to the target 5 by the main pulse to ionize the target 5efficiently to produce ionized gas. Having the minimum density at ornear the plane surface 12 can increase the absorption of main beam bythe target 5 in at least two ways.

First, the minimum density of the target 5 is lower than the density ofa target that is a continuous piece of target material (such as a targetmaterial droplet made of molten tin or a disk shaped target of moltentin). Second, the density gradient 25 places the lowest density portionsof the target 5 at the plane surface 12, which is the plane where theamplified light beam 18 enters the target 5. Because the density of theparticles 20 increases in the “z” direction, most, or all, of theamplified light beam 18 is absorbed by particles 20 that are closer tothe plane surface 12 before the beam 18 reaches and is reflected from aregion of high density within the target 5. Therefore, compared to atarget that has a region of high density closer to the point of impactwith the amplified light beam 18, the target 5 absorbs a higher portionof the energy in the amplified light beam 18. The absorbed light beam 18is used to convert the particles 20 to plasma by ionization. Thus, thedensity gradient 25 also enables more EUV light to be generated.

Second, the target 5 presents a larger area or volume of particles tothe main pulse, enabling increased interaction between the particles 20and the main pulse. Referring to FIGS. 1B and 1C, the target 5 defines alength 30 and a cross-section width 32. The length 30 is the distance inthe “z” direction along which the hemisphere portion 14 extends. Thelength 30 is longer than a similar length in a target that is acontinuous piece of target material because the hemisphere shaped volume10 has a longer extent in the “z” direction. A continuous piece oftarget material is one that has a uniform, or nearly uniform, density inthe direction of propagation of the amplified light beam 18.Additionally, because of the gradient 25, the amplified light beam 18propagates further into the target 5 in the “z” direction whilereflections are kept low. The relatively longer length 30 provides alonger plasma scale length. The plasma scale length for the target 5 canbe, for example, 200 μm, which can be twice the value of the plasmascale length for a disk shaped target made from a continuous piece oftarget material. A longer plasma scale length allows more of theamplified light beam 18 to be absorbed by the target 5.

The cross-section width 32 is the width of the plane surface 12 of thetarget 5. The cross-section interaction width 32 can be, for example,about 200 μm, when the target 5 is generated with a pre-pulse thatoccurs 1000 ns prior to the main pulse, and the pre-pulse has a durationof 150 ps and a wavelength of 1 μm. The cross-section interaction width32 can be about 300 μm when the target 5 is generated with a 50 nsduration CO₂ laser pulse. A pulse of light or radiation has a temporalduration for an amount of time during which a single pulse has anintensity of 50% or more of the maximum intensity of the pulse. Thisduration can also be referred to as the full width at half maximum(FWHM).

Like the length 30, the cross-section width 32 is larger than a similardimension in a target that is made of a continuous, coalesced piece oftarget material (such as a target material droplet made of coalescedmolten metal). Because both the interaction length 30 and theinteraction width 32 are relatively larger than other targets, thetarget 5 also has a larger EUV light emitting volume. The light emittingvolume is the volume in which the particles 20 are distributed and canbe irradiated by the amplified light beam 18. For example, the target 5can have a light emitting volume that is twice that of a disk shapedtarget of molten metal. The larger light emitting volume of the target 5results in generation of greater amounts of EUV light and a higherconversion efficiency (CE) because a higher portion of the targetmaterial (the particles 20) in the target 5 is presented to andirradiated by the amplified light beam 18 and subsequently converted toplasma.

Further, the target 5 does not have a wall or high density region at aback side 4 that could prevent EUV light from being emitted in thedirection of propagation of the main pulse. Thus, the target 5 emits EUVradially outward in all directions, allowing more EUV light to becollected and further increasing the collection efficiency. Moreover,radially isotropic EUV light or substantially isotropic EUV light canprovide improved performance for a lithography tool (not shown) thatuses the EUV light emitted from the target 5 by reducing the amount ofcalibration needed for the tool. For example, if uncorrected, unexpectedspatial variations in EUV intensity can cause overexposure to a waferimaged by the lithography tool. The target 5 can minimize suchcalibration concerns by emitting EUV light uniformly in all directions.Moreover, because the EUV light is radially uniform, errors in alignmentand fluctuations in alignment within the lithography tool or upstreamfrom the lithography tool do not also cause variations in intensity.

FIGS. 2A, 2B, and 3A-3C show exemplary LPP EUV light sources in whichthe target 5 can be used.

Referring to FIG. 2A, an LPP EUV light source 100 is formed byirradiating a target mixture 114 at a target location 105 with anamplified light beam 110 that travels along a beam path toward thetarget mixture 114. The target location 105, which is also referred toas the irradiation site, is within an interior 107 of a vacuum chamber130. When the amplified light beam 110 strikes the target mixture 114, atarget material within the target mixture 114 is converted into a plasmastate that has an element with an emission line in the EUV range toproduce EUV light 106. The created plasma has certain characteristicsthat depend on the composition of the target material within the targetmixture 114. These characteristics can include the wavelength of the EUVlight produced by the plasma and the type and amount of debris releasedfrom the plasma.

The light source 100 also includes a target material delivery system 125that delivers, controls, and directs the target mixture 114 in the formof liquid droplets, a liquid stream, solid particles or clusters, solidparticles contained within liquid droplets or solid particles containedwithin a liquid stream. The target mixture 114 can also includeimpurities such as non-target particles. The target mixture 114 isdelivered by the target material delivery system 125 into the interior107 of the chamber 130 and to the target location 105.

The light source 100 includes a drive laser system 115 that produces theamplified light beam 110 due to a population inversion within the gainmedium or mediums of the laser system 115. The light source 100 includesa beam delivery system between the laser system 115 and the targetlocation 105, the beam delivery system including a beam transport system120 and a focus assembly 122. The beam transport system 120 receives theamplified light beam 110 from the laser system 115, and steers andmodifies the amplified light beam 110 as needed and outputs theamplified light beam 110 to the focus assembly 122. The focus assembly122 receives the amplified light beam 110 and focuses the beam 110 tothe target location 105.

In some implementations, the laser system 115 can include one or moreoptical amplifiers, lasers, and/or lamps for providing one or more mainpulses and, in some cases, one or more pre-pulses. Each opticalamplifier includes a gain medium capable of optically amplifying thedesired wavelength at a high gain, an excitation source, and internaloptics. The optical amplifier may or may not have laser mirrors or otherfeedback devices that form a laser cavity. Thus, the laser system 115produces an amplified light beam 110 due to the population inversion inthe gain media of the laser amplifiers even if there is no laser cavity.Moreover, the laser system 115 can produce an amplified light beam 110that is a coherent laser beam if there is a laser cavity to provideenough feedback to the laser system 115. The term “amplified light beam”encompasses one or more of: light from the laser system 115 that ismerely amplified but not necessarily a coherent laser oscillation andlight from the laser system 115 that is amplified (externally or withina gain medium in the oscillator) and is also a coherent laseroscillation.

The optical amplifiers in the laser system 115 can include as a gainmedium a filling gas that includes CO₂ and can amplify light at awavelength of between about 9100 and about 11000 nm, and in particular,at about 10.6 μm, at a gain greater than or equal to 1000. In someexamples, the optical amplifiers amplify light at a wavelength of 10.59μm. Suitable amplifiers and lasers for use in the laser system 115 caninclude a pulsed laser device, for example, a pulsed, gas-discharge CO₂laser device producing radiation at about 9300 nm or about 10600 nm, forexample, with DC or RF excitation, operating at relatively high power,for example, 10 kW or higher and high pulse repetition rate, forexample, 50 kHz or more. The optical amplifiers in the laser system 115can also include a cooling system such as water that can be used whenoperating the laser system 115 at higher powers.

FIG. 2B shows a block diagram of an example drive laser system 180. Thedrive laser system 180 can be used as the drive laser system 115 in thesource 100. The drive laser system 180 includes three power amplifiers181, 182, and 183. Any or all of the power amplifiers 181, 182, and 183can include internal optical elements (not shown). The power amplifiers181, 182, and 183 each include a gain medium in which amplificationoccurs when pumped with an external electrical or optical source.

Light 184 exits from the power amplifier 181 through an output window185 and is reflected off a curved mirror 186. After reflection, thelight 184 passes through a spatial filter 187, is reflected off of acurved mirror 188, and enters the power amplifier 182 through an inputwindow 189. The light 184 is amplified in the power amplifier 182 andredirected out of the power amplifier 182 through an output window 190as light 191. The light 191 is directed toward the amplifier 183 withfold mirrors 192 and enters the amplifier 183 through an input window193. The amplifier 183 amplifies the light 191 and directs the light 191out of the amplifier 183 through an output window 194 as an output beam195. A fold mirror 196 directs the output beam 195 upwards (out of thepage) and toward the beam transport system 120.

The spatial filter 187 defines an aperture 197, which can be, forexample, a circular opening through which the light 184 passes. Thecurved mirrors 186 and 188 can be, for example, off-axis parabolamirrors with focal lengths of about 1.7 m and 2.3 m, respectively. Thespatial filter 187 can be positioned such that the aperture 197coincides with a focal point of the drive laser system 180. The exampleof FIG. 2B shows three power amplifiers. However, more or fewer poweramplifiers can be used.

Referring again to FIG. 2A, the light source 100 includes a collectormirror 135 having an aperture 140 to allow the amplified light beam 110to pass through and reach the target location 105. The collector mirror135 can be, for example, an ellipsoidal mirror that has a primary focusat the target location 105 and a secondary focus at an intermediatelocation 145 (also called an intermediate focus) where the EUV light canbe output from the light source 100 and can be input to, for example, anintegrated circuit beam positioning system tool (not shown). The lightsource 100 can also include an open-ended, hollow conical shroud 150(for example, a gas cone) that tapers toward the target location 105from the collector mirror 135 to reduce the amount of plasma-generateddebris that enters the focus assembly 122 and/or the beam transportsystem 120 while allowing the amplified light beam 110 to reach thetarget location 105. For this purpose, a gas flow can be provided in theshroud that is directed toward the target location 105.

The light source 100 can also include a master controller 155 that isconnected to a droplet position detection feedback system 156, a lasercontrol system 157, and a beam control system 158. The light source 100can include one or more target or droplet imagers 160 that provide anoutput indicative of the position of a droplet, for example, relative tothe target location 105 and provide this output to the droplet positiondetection feedback system 156, which can, for example, compute a dropletposition and trajectory from which a droplet position error can becomputed either on a droplet by droplet basis or on average. The dropletposition detection feedback system 156 thus provides the dropletposition error as an input to the master controller 155. The mastercontroller 155 can therefore provide a laser position, direction, andtiming correction signal, for example, to the laser control system 157that can be used, for example, to control the laser timing circuitand/or to the beam control system 158 to control an amplified light beamposition and shaping of the beam transport system 120 to change thelocation and/or focal power of the beam focal spot within the chamber130.

The target material delivery system 125 includes a target materialdelivery control system 126 that is operable in response to a signalfrom the master controller 155, for example, to modify the release pointof the droplets as released by a target material supply apparatus 127 tocorrect for errors in the droplets arriving at the desired targetlocation 105.

Additionally, the light source 100 can include a light source detector165 that measures one or more EUV light parameters, including but notlimited to, pulse energy, energy distribution as a function ofwavelength, energy within a particular band of wavelengths, energyoutside of a particular band of wavelengths, and angular distribution ofEUV intensity and/or average power. The light source detector 165generates a feedback signal for use by the master controller 155. Thefeedback signal can be, for example, indicative of the errors inparameters such as the timing and focus of the laser pulses to properlyintercept the droplets in the right place and time for effective andefficient EUV light production.

The light source 100 can also include a guide laser 175 that can be usedto align various sections of the light source 100 or to assist insteering the amplified light beam 110 to the target location 105. Inconnection with the guide laser 175, the light source 100 includes ametrology system 124 that is placed within the focus assembly 122 tosample a portion of light from the guide laser 175 and the amplifiedlight beam 110. In other implementations, the metrology system 124 isplaced within the beam transport system 120. The metrology system 124can include an optical element that samples or re-directs a subset ofthe light, such optical element being made out of any material that canwithstand the powers of the guide laser beam and the amplified lightbeam 110. A beam analysis system is formed from the metrology system 124and the master controller 155 since the master controller 155 analyzesthe sampled light from the guide laser 175 and uses this information toadjust components within the focus assembly 122 through the beam controlsystem 158.

Thus, in summary, the light source 100 produces an amplified light beam110 that is directed along the beam path to irradiate the target mixture114 at the target location 105 to convert the target material within themixture 114 into plasma that emits light in the EUV range. The amplifiedlight beam 110 operates at a particular wavelength (that is alsoreferred to as a source wavelength) that is determined based on thedesign and properties of the laser system 115. Additionally, theamplified light beam 110 can be a laser beam when the target materialprovides enough feedback back into the laser system 115 to producecoherent laser light or if the drive laser system 115 includes suitableoptical feedback to form a laser cavity.

Referring to FIG. 3A, a top plan view of an exemplary optical imagingsystem 300 is shown. The optical imaging system 300 includes an LPP EUVlight source 305 that provides EUV light to a lithography tool 310. Thelight source 305 can be similar to, and/or include some or all of thecomponents of, the light source 100 of FIGS. 2A and 2B. As discussedbelow, the target 5 can be used in the light source 305 to increase theamount of light emitted by the light source 305.

The light source 305 includes a drive laser system 315, an opticalelement 322, a pre-pulse source 324, a focusing assembly 326, a vacuumchamber 340, and an EUV collecting optic 346. The EUV collecting optic346 directs the EUV light emitted by converting the target 5 to plasmato the lithography tool 310. The EUV collection optic 346 can be themirror 135 (FIG. 2A).

Referring also to FIGS. 3B-3E, the light source 305 also includes atarget material delivery apparatus 347 that produces a stream of targetmaterial 348. The stream of target material 348 can include targetmaterial in any form, such as liquid droplets, a liquid stream, solidparticles or clusters, solid particles contained within liquid dropletsor solid particles contained within a liquid stream. In the discussionbelow, the target material stream 348 includes target material droplets348. In other examples, the target material stream can include targetmaterial of other forms.

The target material droplets travel along the “x” direction from thetarget material delivery apparatus 347 to a target location 342 in thevacuum chamber 340. The drive laser system 315 produces an amplifiedlight beam 316. The amplified light beam 316 can be similar to theamplified light beam 18 of FIGS. 1A-1C, or the amplified light beam 110of FIGS. 2A and 2B, and can be referred to as a main pulse or a mainbeam. The amplified light beam 316 has an energy sufficient to convertthe particles 20 in the target 5 into plasma that emits EUV light.

In some implementations, the drive laser system 315 can be a dual-stagemaster oscillator and power amplifier (MOPA) system that uses carbondioxide (CO₂) amplifiers within the master oscillator and poweramplifier, and the amplified light beam 316 can be a 130 ns duration,10.6 μm wavelength CO₂ laser light pulse generated by the MOPA. In otherimplementations, the amplified light beam 316 can be a CO₂ laser lightpulse that has a duration of less than 50 ns.

The pre-pulse source 324 emits a pulse of radiation 317. The pre-pulsesource 324 can be, for example, a Q-switched Nd:YAG laser, and the pulseof radiation 317 can be a pulse from the Nd.YAG laser. The pulse ofradiation 317 can have a duration of 10 ns and a wavelength of 1.06 μm,for example.

In the example shown in FIG. 3A, the drive laser system 315 and thepre-pulse source 324 are separate sources. In other implementations,they can be a part of the same source. For example, both the pulse ofradiation 317 and the amplified light beam 316 can be generated by thedrive laser system 315. In such an implementation, the drive lasersystem 315 can include two CO₂ seed laser subsystems and one amplifier.One of the seed laser subsystems can produce an amplified light beamhaving a wavelength of 10.26 μm, and the other seed laser subsystem canproduce an amplified light beam having a wavelength of 10.59 μm. Thesetwo wavelengths can come from different lines of the CO₂ laser. Bothamplified light beams from the two seed laser subsystems are amplifiedin the same power amplifier chain and then angularly dispersed to reachdifferent locations within the chamber 340. In one example, theamplified light beam with the wavelength of 10.26 μm is used as thepre-pulse 317, and the amplified light beam with the wavelength of 10.59μm is used as the amplified light beam 316. In other examples, otherlines of the CO₂ laser, which can generate different wavelengths, can beused to generate the two amplified light beams (one of which is thepulse of radiation 317 and the other of which is the amplified lightbeam 316).

Referring again to FIG. 3A, the optical element 322 directs theamplified light beam 316 and the pulse of radiation 317 from thepre-pulse source 324 to the chamber 340. The optical element 322 is anyelement that can direct the amplified light beam 316 and the pulse ofradiation 317 along similar paths and deliver the amplified light beam316 and the pulse of radiation 317 to the chamber 340. In the exampleshown in FIG. 3A, the optical element 322 is a dichroic beamsplitterthat receives the amplified light beam 316 and reflects it toward thechamber 340. The optical element 322 receives the pulse of radiation 317and transmits the pulses toward the chamber 340. The dichroicbeamsplitter has a coating that reflects the wavelength(s) of theamplified light beam 316 and transmits the wavelength(s) of the pulse ofradiation 317. The dichroic beamsplitter can be made of, for example,diamond.

In other implementations, the optical element 322 is a mirror thatdefines an aperture (not shown). In this implementation, the amplifiedlight beam 316 is reflected from the mirror surface and directed towardthe chamber 340, and the pulses of radiation pass through the apertureand propagate toward the chamber 340.

In still other implementations, a wedge-shaped optic (for example, aprism) can be used to separate the main pulse 316, the pre-pulse 317,and the pre-pulse 318 into different angles, according to theirwavelengths. The wedge-shaped optic can be used in addition to theoptical element 322, or it can be used as the optical element 322. Thewedge-shaped optic can be positioned just upstream (in the “−z”direction) of the focusing assembly 326.

Additionally, the pulse of radiation 317 can be delivered to the chamber340 in other ways. For example, the pulse 317 can travel through opticalfibers that deliver the pulses 317 and 318 to the chamber 340 and/or thefocusing assembly 326 without the use of the optical element 322 orother directing elements. In these implementations, the fiber can bringthe pulse of radiation 317 directly to an interior of the chamber 340through an opening formed in a wall of the chamber 340.

Regardless of how the amplified light beam 316 and the pulses ofradiation 317 and 318 are directed toward the chamber 340, the amplifiedlight beam 316 is directed to a target location 342 in the chamber 340.The pulse of radiation 317 is directed to a location 341. The location341 is displaced from the target location 342 in the “−x” direction.

The amplified light beam 316 from the drive laser system 315 isreflected by the optical element 322 and propagates through the focusingassembly 326. The focusing assembly 326 focuses the amplified light beam316 onto the target location 342. The pulse of radiation 317 from thepre-pulse source 324 passes through the optical element 322 and throughthe focusing assembly 216 to the chamber 340. The pulse of radiation 317propagates to the location 341 in the chamber 340 that is in the “−x”direction relative to the target location 342. The displacement betweenthe location 342 and the location 341 allows the pulse of radiation 317to irradiate a target material droplet to convert the droplet to thehemisphere shaped target 5 before the target 5 reaches the targetlocation 342 without substantially ionizing the target 5. In thismanner, the hemisphere shaped target 5 can be a pre-formed target thatis formed at a time before the target 5 enters the target location 342.

In greater detail and referring also to FIGS. 3B and 3C, the targetlocation 342 is a location inside of the chamber 340 that receives theamplified light beam 316 and a droplet in the stream of target materialdroplets 348. The target location 342 is also a location that ispositioned to maximize an amount of EUV light delivered to the EUVcollecting optic 346. For example, the target location 342 can be at afocal point of the EUV collecting optic 346. FIGS. 3B and 3C show topviews of the chamber 340 at times t₁ and t₂, respectively, with time=t₁occurring before time=t₂. In the example shown in FIGS. 3B and 3C, theamplified light beam 316 and the pulsed beam of radiation 317 occur atdifferent times and are directed toward different locations within thechamber 340.

The stream 348 travels in the “x” direction from the target materialsupply apparatus 347 to the target location 342. The stream of targetmaterial droplets 348 includes the target material droplets 348 a, 348b, and 348 c. At a time=t₁ (FIG. 3B), the target material droplets 348 aand 348 b travel in the “x” direction from the target material supplyapparatus 347 to the target location 342. The pulsed beam of radiation317 irradiates the target material droplet 348 a at the time “t₁” at thelocation 341, which is displaced in the “−x” direction from the targetlocation 342. The pulsed beam of radiation 317 transforms the targetmaterial droplet 348 b into the hemisphere target 5. At the time=t₂(FIG. 3C), the amplified light beam 316 irradiates the target 5 andconverts the particles 20 of target material into EUV light.

Referring to FIG. 4, an exemplary process 400 for generating thehemisphere shaped target 5 is shown. The process 400 can be performedusing the target material supply apparatus 127 (FIG. 2A) or the targetmaterial supply apparatus 347 (FIGS. 3B-3E).

An initial target material is released toward a target location (410).Referring also to FIGS. 3B and 3C, the target material droplet 348 a isreleased from the target material supply apparatus 347 and travelstoward the target location 342. The initial target material is a targetmaterial droplet that emerges or is released from the target materialsupply apparatus 347 as a droplet. The initial target material dropletis a droplet that has not been transformed or altered by a pre-pulse.The initial target material droplet can be a coalesced sphere orsubstantially spherical piece of molten metal that can be considered asa continuous piece of target material. The target material droplet 348 aprior to the time “t₁” is an example of an initial target material inthis example.

A first amplified light beam is directed toward the initial targetmaterial to generate a collection of pieces of target materialdistributed in a hemisphere shaped volume (420) without substantiallyionizing the initial target material. The collection of pieces of targetmaterial can be the particles 20 (FIGS. 1A-1C), which are distributed inthe hemisphere shaped volume 10. The first amplified light beam can bethe pulsed light beam 317 emitted from the source 324 (FIGS. 3A, 3D, and3E). The first amplified light beam can be referred to as the“pre-pulse.” The first amplified light beam is a pulse of light that hasan energy and/or pulse duration sufficient to transform the targetmaterial droplet 348 a from a droplet that is a continuous or coalescedsegment or piece of molten target material into the target 5, which is ahemisphere shaped distribution of particles 20.

The first amplified light beam can be, for example, a pulse of lightthat has a duration of 130 ns and a wavelength of 1 μm. In anotherexample, first amplified light beam can be a pulse of light that has aduration of 150 ps, a wavelength of 1 μm, an energy of 10 milliJoules(mJ), a 60 μm focal spot, and an intensity of 2×10¹²W/cm². The energy,wavelength, and/or duration of the first amplified light beam areselected to transform the target material droplet into the hemisphereshaped target 5. In some implementations, the first amplified light beamincludes more than one pulse. For example, the first amplified lightbeam can include two pulses, separated from each other in time, andhaving different energies and durations. FIG. 9 shows an example inwhich the first amplified light beam includes more than one pulse.Further, the first amplified light beam can be a single pulse that has ashape (energy or intensity as a function of time) to provide an effectthat is similar to that achieved by multiple pre-pulses. The secondamplified light beam has energy sufficient to convert the targetmaterial droplet into a collection of pieces.

A second amplified light beam is directed toward the collection ofpieces to convert the particles 20 to plasma that emits EUV light (430).The second amplified light beam can be referred to as the “main pulse.”The amplified light beam 316 of FIG. 3A is an example of a secondamplified light beam. The amplified light beam 316 has sufficient energyto convert all or most of the particles 20 of the target 5 into plasmathat emits EUV light.

Referring to FIG. 5, an example of a waveform 500 that can be used totransform a target material droplet into a hemisphere shaped target isshown. FIG. 5 shows the amplitude of the waveform 500 as a function oftime. The waveform 500 shows a representation of the collection ofamplified light beams that strike a particular target material dropletin a single cycle of operation of the EUV light source. A cycle ofoperation is a cycle that emits a pulse or burst of EUV light. Thewaveform 500 also can be referred to as a laser train 500 or a pulsetrain 500. In the waveform 500, the collection of amplified light beamsincludes a pre-pulse 502 and a main pulse 504.

The pre-pulse 502 begins at time t=0, and the main pulse 504 begins at atime t=1000 ns. In other words, the main pulse 504 occurs 1000 ns afterthe pre-pulse 502. In the waveform 500, the pre-pulse 502 can have awavelength of 1.0 μm, a duration of 150 ps, an energy of 10 mJ, a focalspot 60 μm in diameter, and an intensity of 2×10¹² W/cm². This is anexample of one implementation of the waveform 500. Other parametervalues can be used, and the parameter values of the pre-pulse 502 canvary by a factor of 5 as compared to this example. For example, in someimplementations, the pre-pulse 502 can have a duration of 5-20 ps, andan energy of 1-20 mJ. The main pulse 504 can have a wavelength of 5-11μm, a pulse duration of 15-200 ns, a focus spot size of 50-300 μm, andan intensity of 3×10⁹ to 8×10¹° W/cm². For example, the main pulse 504can have a wavelength of 10.59 μm and a pulse duration of 130 ns. Inanother example, the main pulse can have a wavelength of 10.59 μm and apulse duration of 50 ns or less.

In addition to the times t=0 and t=1000 ns, the times t₁-t₄ are alsoshown on the time axis. The time t₁ is shortly before the pre-pulse 502occurs. The time t₂ is after the pre-pulse 502 ends and before the mainpulse 504 begins. The time t3 occurs shortly before the main pulse 504,and the time t4 occurs after the main pulse 504. The times t₁-t₄ areused in the discussion below, with respect to FIGS. 6A-6D, of atransformation of a target material droplet to a hemisphere shapedtarget using the waveform 500.

Although the waveform 500 is shown as a continuous waveform in time, thepre-pulse 502 and main pulse 504 that make up the waveform 500 can begenerated by different sources. For example, the pre-pulse 502 can be apulse of light generated by the pre-pulse source 324, and the main pulse504 can be generated by the drive laser system 315. When the pre-pulse502 and the main pulse 504 are generated by separate sources that are indifferent locations relative to the chamber 340 (FIG. 3A), the pre-pulse502 and the main pulse 504 can be directed to the chamber 340 with theoptical element 322.

Referring also to FIGS. 6A-6D, interactions between a target materialdroplet 610 and the waveform 500 that transform the target materialdroplet 610 into a hemisphere shaped target 614 are shown. A targetsupply apparatus 620 releases a stream of target material droplets 622from an orifice 624. The target material droplets 622 travel in the “x”direction toward a target location 626. FIGS. 6A-6D show the targetsupply apparatus 620 and the droplet stream 622 at the times t=t₁, t=t₂,t=t₃, and t=t₄, respectively. FIG. 5 also shows the times t=t₁ throught=t₄ relative to the waveform 500.

Referring to FIG. 6A, the pre-pulse 502 approaches the target materialdroplet 610. The target material droplet 610 is a droplet of targetmaterial. The target material can be molten metal, such as molten tin.The target material droplet 610 is a continuous segment or piece oftarget material that has a uniform density in the “z” direction (thedirection of propagation of the waveform 500). The cross-sectional sizeof a target material droplet can be, for example, between 20-40 μm. FIG.7A shows the density of the target material droplet 610 as a function ofposition along the “z” direction. As shown in FIG. 7A, compared to freespace, the target material droplet 610 presents a steep increase indensity to the waveform 500.

The interaction between the pre-pulse 502 and the target materialdroplet 610 forms a collection of pieces of target material 612 that arearranged in a geometric distribution. The pieces of target material 612are distributed in a hemisphere shaped volume that extends outward froma plane surface 613 in the “x” and “z” direction. The pieces of targetmaterial 612 can be a mist of nano- or micro-particles, separate piecesof molten metal, or a cloud of atomic vapor. The pieces of targetmaterial can be 1-10 μm in diameter.

A purpose of the interaction between the pre-pulse 502 and the targetmaterial droplet 610 is to form a target that has a spatial extent thatis larger than the diameter of the main pulse 504 but withoutsubstantially ionizing the target. In this manner, as compared to asmaller target, the created target presents more target material to themain beam and can use more of the energy in the main pulse 504. Thepieces of target material 612 have a spatial extent in the x-y and x-zplanes that is larger than the extent of the target material droplet 610in the x-y and x-z planes.

As time passes, the collection of pieces 612 travels in the “x”direction toward the target location 626. The collection of pieces 612also expands in the “x” and “z” directions while moving toward thetarget location 626. The amount of spatial expansion depends on theduration and intensity of the pre-pulse 502, as well as the amount oftime over which the collection of pieces 612 is allowed to expand. Thedensity of the collection of pieces 612 decreases as time passes,because the pieces spread out. A lower density generally allows anoncoming light beam to be absorbed by more of the material in a volume,and a high density can prevent or reduce the amount of light absorbedand the amount of EUV light produced. A wall of high density throughwhich light cannot pass or be absorbed and is instead reflected is the“critical density.” However, the most efficient absorption by a materialcan occur near but below the critical density. Thus, it can bebeneficial to for the target 614 to be formed by allowing the collectionof pieces 614 to expand over a finite time period that is long enough toallow the collection of pieces 613 to expand spatially without being solong that the density of the pieces decreases to a point where theefficiency of laser absorption decreases. The finite time period can bethe time between the pre-pulse 502 and the main pulse 504 and can be,for example, about 1000 ns.

Referring also to FIGS. 8A and 8B, examples of the spatial expansion ofthe collection of pieces 612 as a function of time after the pre-pulsestrikes a target material droplet for two different pre-pulses areshown, with FIG. 8A showing an example for a pre-pulse similar to thepre-pulse 502. The time after the pre-pulse strikes a target materialdroplet can be referred to as the delay time. FIG. 8A shows the size ofthe collection of pieces 612 as a function of delay time when thepre-pulse has a wavelength of 1.0 μm, a duration of 150 ps, an energy of10 mJ, a focal spot 60 μm in diameter, and an intensity of 2×10¹² W/cm².FIG. 8B shows the size of the collection of pieces 612 as a function ofdelay time when the pre-pulse has a wavelength of 1.0 μm, a duration of150 ps, an energy of 5 mJ, a focal spot 60 μm in diameter, and anintensity of 1×10¹² W/cm². Comparing FIG. 8A to FIG. 8B shows that thecollection of pieces 612 expands more rapidly in the vertical directions(x/y) when struck by the more energetic and more intense pre-pulse ofFIG. 8A.

Referring again to, FIG. 6C the target material droplet 610 and thestream of droplets 622 are shown at the time=t₃. At the time=t₃, thecollection of target material pieces 612 has expanded into thehemisphere shaped target 614 and arrives at the target location 626. Themail pulse 504 approaches the hemisphere shaped target 614.

FIG. 7B shows the density of the hemisphere shaped target 614 justbefore the main pulse 504 reaches the target 614. The density isexpressed as density gradient 705 that is density of particles 612 inthe target 614 a function of position in the “z” direction, with z=0being the plane surface 613. As shown, the density is minimum at theplane surface 613 and increases in the “z” direction. Because thedensity is at a minimum at the plane surface 613, and the minimumdensity is lower than that of the target material droplet 610, comparedto the target material droplet 610, the main pulse 504 enters the target614 relatively easily (less of the main pulse 504 is absorbed).

As the main beam 504 travels in the target 614, the particles 612 absorbthe energy in the main beam 504 and are converted to plasma that emitsEUV light. The density of the target 614 increases in the direction ofpropagation “z” and can increase to an amount where the main beam 504cannot penetrate and is instead reflected. The location in the target614 with such a density is the critical surface (not shown). However,because the density of the target 614 is initially relatively low, amajority, most, or all, of the main beam 504 is absorbed by theparticles 615 prior to reaching the critical surface. Thus, the densitygradient provides a target that is favorable for EUV light generation.

Additionally, because the hemisphere shaped target 614 does not have awall of high density, the EUV light 618 is radially emitted from thetarget 614 in all directions. This is unlike a disk shaped target orother target with a higher density, where the interaction between themain pulse and the target generates plasma and a shock wave that blowsoff some of the target as dense target material in the direction ofpropagation of the main pulse 504. The blown off material reduces theamount of material available for conversion to plasma and also absorbssome of the EUV light emitted in the forward (“z”) direction. As aresult, the EUV light is emitted over 2π steradians, and only half ofthe EUV light is available for collection.

However, the hemisphere shaped target 614 allows collection of EUV lightin all directions (4π steradians). After the main pulse 504 irradiatesthe hemisphere shaped target 614, there is negligible or no dense targetmaterial left in the hemisphere shaped target 614, and the EUV light 618is able to escape the hemisphere shaped target 614 radially in alldirections. In effect, there is very little matter present to block orabsorb the EUV light 618 and prevent it from escaping. In someimplementations, the EUV light 618 can be isotropic (uniform intensity)in all directions.

Thus, the hemisphere shaped target 614 provides additional EUV light byallowing EUV light 619, which is generated in the forward direction, toescape the hemisphere shaped target 614. Because the hemisphere shapedtarget 614 emits EUV light in all directions, a light source that usesthe hemisphere shaped target 614 can have increased conversionefficiency (CE) as compared to a light source that uses a target thatemits light over only 2π steradians. For example, when measured over 2πsteradians, a hemisphere shaped target that is irradiated with a MOPACO₂ main pulse having a duration of 130 ns can have a conversionefficiency of 3.2%, meaning that 3.2% of the CO₂ main pulse is convertedinto EUV light. When the hemisphere shaped target is irradiated with aMOPA CO₂ main pulse having a duration of 50 ns, the conversionefficiency is 5% based on measuring the EUV light emitted over 2πsteradians. When the EUV light is measured over 4π steradians, theconversion efficiency is doubled because the amount of EUV light emittedfrom the target is doubled. Thus, the conversion efficiency for the twomain pulses becomes 6.4% and 10%, respectively.

In the example of FIGS. 6A-6D, the waveform 500, which has a delay timeof 1000 ns between the pre-pulse 502 and the main pulse 504, is used totransform the target material droplet 610 into the hemisphere shapedtarget 614. However, other waveforms with other delay times can be usedfor the transformation. For example, the delay between the pre-pulse 502and the main pulse 504 can be between 200 ns and 1600 ns. A longer delaytime provides a target with a larger spatial extent (volume) and a lowerdensity of target material. A shorter delay time provides a target witha smaller spatial extent (volume) and a higher density of targetmaterial.

FIG. 9 shows another exemplary waveform 900 that, when applied to atarget material droplet, transforms the target material droplet to ahemisphere shaped target. The waveform 900 includes a first pre-pulse902, a second pre-pulse 904, and a main pulse 906. The first pre-pulse902 and the second pre-pulse 904 can be collectively considered as thefirst amplified light beam, and the main pulse 906 can be considered asthe second amplified light beam. The first pre-pulse 902 occurs at timet=0, the second pre-pulse 904 occurs 200 ns later at time t=200 ns, andthe main pulse 906 occurs at time t=1200 ns, 1200 ns after the firstpre-pulse 902. In the example of FIG. 9, the first pre-pulse 502 has aduration of 1-10 ns, and the second pre-pulse 504 has a duration of lessthan 1 ns. For example, the duration of the second pre-pulse 504 can be150 ps. The first pre-pulse 502 and the second pre-pulse 504 can have awavelength of 1 μm. The main pulse 506 can be a pulse from a CO₂ laserthat has a wavelength of 10.6 μm and a duration of 130 ns or 50 ns.

FIGS. 10A-10D show the waveform 900 interacting with a target materialdroplet 1010 to transform the target material droplet 1010 into ahemisphere shaped target 1018. FIGS. 10A-10D show times t=t₁ to t₄,respectively. Times t=t₁ to t₄ are shown relative to the waveform 900 onFIG. 9. The time t=t₁ occurs just before the first pre-pulse 502, andthe time t=t₂ occurs just before the second pre-pulse 504. The time t=t₃occurs just before the main pulse 506, and the time t=t4 occurs justafter the main pulse 506.

Referring to FIG. 10A, a target material supply apparatus 1020 releasesa stream of target material droplets 1022. The stream 1022 travels fromthe target material supply apparatus 1020 to a target location 1026. Thestream 1022 includes target material droplets 1010 and 1011. The firstpre-pulse 502 approaches and strikes the target material droplet 1010.The cross-sectional size of a target material droplet can be, forexample, between 20-40 μm. Referring also to FIG. 11A, the densityprofile 1100 of the target material droplet 1010 is uniform in thedirection of propagation “z” of the pre-pulse 502, and the targetmaterial droplet 1010 presents a steep density transition to thepre-pulse 502.

The interaction between the first pre-pulse 502 and the target materialdroplet 1010 produces a short-scale plume 1012 (FIG. 10B) on a side ofthe target material droplet 1010 that faces the oncoming first pre-pulse902. The plume 1012 can be a cloud of particles of the target materialthat is formed on or adjacent to the surface of the target materialdroplet 1010. As the target material droplet 1010 travels toward thetarget location 1026, the target material droplet 1010 can increase insize in the vertical “x” direction and decrease in size in the “z”direction. Together, the plume 1012 and the target material droplet 1010can be considered as an intermediate target 1014. The intermediatetarget 1014 receives the second pre-pulse 504.

Referring also to FIG. 11B, at the time t=t₂, the intermediate target1014 has a density profile 1102. The density profile includes a densitygradient 1105 that corresponds to the portion of the intermediate target1014 that is the plume 1012. The density gradient 1105 is minimum at alocation 1013 (FIG. 10B) where the second pre-pulse 504 initiallyinteracts with the plume 1012. The density gradient 1105 increases inthe direction “z” until the plume 1012 ends and the target material 1010is reached. Thus, the first pre-pulse 502 acts to create an initialdensity gradient that includes densities that are lower than thosepresent in the target material droplet 1010, thereby enabling theintermediate target 1014 to absorb the second pre-pulse 504 more readilythan the target material droplet 1010. The second pre-pulse 504 strikesthe intermediate target 1014 and generates a collection of pieces oftarget material 1015. The interaction between the intermediate target1014 and the second pre-pulse 504 generates the collection of pieces1015, as shown in FIG. 10C. As time passes, the collection of pieces oftarget material 1015 continues to travel in the “x” direction toward thetarget location 1026. The collection of pieces of target material 1015forms a volume, and the volume increases as the pieces expand with thepassage of time. Referring to FIG. 10D, the collection of pieces expandsfor 1000 ns after the second pre-pulse 502 strikes the intermediatetarget 1014, and the expanded collection of pieces forms the hemisphereshaped target 1018. The hemisphere shaped target 1018 enters the targetlocation 1016 at time t=t4. The hemisphere shaped target 1018 hasdensity that is at a minimum at a surface plane 1019, which receives themain pulse 506, and increases in the “z” direction.

The density profile 1110 of the hemisphere shaped target 1018 at a timejust before the main pulse 506 strikes the target 1018 is shown in FIG.11C. The hemisphere shaped target 1018 has a gentle gradient that is ata minimum at the surface plane 1019 that receives the main pulse 506.Thus, like the hemisphere target 614, the hemisphere target 1018 absorbsthe main pulse 506 readily and emits EUV light 1030 in all directions.As compared to the hemisphere target 614, the maximum density of thetarget 1018 is lower and the gradient is less steep.

Other implementations are within the scope of the following claims. Forexample, the shape of the target can vary from a hemisphere that has arounded surface. The hemisphere shaped portion 14 of the hemisphereshaped target 5 can have one or more sides that are flattened instead ofbeing rounded. In addition to, or alternatively to, being dispersedthroughout the hemisphere shaped target 5, the particles 20 can bedispersed on a surface of the hemisphere shaped target 5.

What is claimed is:
 1. A method comprising: releasing an initial targetmaterial toward a target location, the target material comprising amaterial that emits extreme ultraviolet (EUV) light when converted toplasma; directing a first amplified light beam toward the initial targetmaterial, the first amplified light beam having an energy sufficient toform a collection of pieces of target material from the initial targetmaterial, each of the pieces being smaller than the initial targetmaterial and being spatially distributed throughout a hemisphere shapedvolume, the hemisphere shaped volume comprising a rounded portion havinga circumference and a planar portion extending throughout a regiondefined by the circumference; and directing a second amplified lightbeam toward the collection of pieces to convert the pieces of targetmaterial to plasma that emits EUV light.
 2. The method of claim 1,wherein EUV light is emitted from the hemisphere shaped volume in alldirections.
 3. The method of claim 1, wherein EUV light is emitted fromthe hemisphere shaped volume isotropically.
 4. The method of claim 1,wherein the initial target material comprises a metal, and thecollection of pieces comprises pieces of the metal.
 5. The method ofclaim 4, wherein the metal comprises tin.
 6. The method of claim 1,wherein the hemisphere shaped volume defines a longitudinal axis along adirection that is parallel to a direction of propagation of the secondamplified light beam and a transverse axis along a direction that istransverse to the direction of propagation of the second amplified lightbeam, and directing the second amplified light beam toward thecollection of pieces comprises penetrating into the hemisphere shapedvolume along the longitudinal axis.
 7. The method of claim 1, whereinthe majority of the pieces in the collection of pieces are converted toplasma.
 8. The method of claim 1, wherein the first amplified light beamcomprises a pulse of light having a duration of 150 ps and a wavelengthof 1 μm.
 9. The method of claim 1, wherein the first amplified lightbeam comprises a pulse of light having a duration of less than 150 psand a wavelength of 1 μm.
 10. The method of claim 1, wherein the firstamplified light beam comprises two pulses of light that are temporallyseparated from each other.
 11. The method of claim 10, wherein the twopulses comprise a first pulse of light and a second pulse of light, thefirst pulse of light having a duration of 1 ns to 10 ns, and the secondpulse of light having a duration of less than 1 ns.
 12. The method ofclaim 1, wherein the first amplified light beam has an energy that isinsufficient to convert the target material to plasma, and the secondamplified light beam has an energy that is sufficient to convert thetarget material to plasma.
 13. The method of claim 1, wherein a densityof the pieces of target material increases along a direction that isparallel to a direction of propagation of the second amplified lightbeam.
 14. The method of claim 1, wherein the pieces of target materialin the hemisphere shaped volume have a diameter of 1-10 μm.
 15. A targetfor an extreme ultraviolet (EUV) light source, the target comprising:pieces of a target material distributed throughout a hemisphere shapedvolume comprising a rounded portion having a circumference, the targetmaterial comprising a material that emits EUV light when converted toplasma; and the target further comprising a plane surface adjacent tothe rounded portion of the hemisphere shaped volume and defining a frontboundary of the hemisphere shaped volume, the front boundary extendingthroughout a region defined by the circumference and being positioned toface a source of an amplified light beam, wherein the rounded portion ofthe hemisphere shaped volume extends away from the source of theamplified light beam.
 16. The target of claim 15, wherein the hemisphereshaped volume has a cross-sectional diameter in a direction that istransverse to a direction of propagation of the amplified light beam,and a maximum of the cross-sectional diameter is at the plane surface.17. The target of claim 15, wherein a density of the pieces of thetarget material in the hemisphere shaped volume increases along adirection that is parallel to a direction of propagation of theamplified light beam.
 18. The target of claim 15, wherein at least someof the pieces are individual pieces that are physically separated fromeach other.
 19. The target of claim 15, wherein, when the hemisphereshaped volume is irradiated with an amplified light beam havingsufficient energy to convert the individual pieces of the targetmaterial to plasma, the hemisphere shaped target emits EUV light in alldirections.
 20. An extreme ultraviolet (EUV) light source comprising: afirst source that produces a pulse of light; a second source thatproduces an amplified light beam; a target material delivery system; achamber coupled to the target material delivery system; and a steeringsystem that steers the amplified light beam toward a target location inthe chamber that receives a target material droplet from the targetmaterial delivery system, the target material droplet comprising amaterial that emits EUV light after being converted to plasma, whereinthe target material droplet forms a target when struck by the pulse oflight, the target comprising a hemisphere shaped volume having pieces ofthe target material throughout the volume, the hemisphere shaped volumecomprising a rounded portion having a circumference and a plane surfacethat extends throughout a region defined by the circumference, the planesurface positioned between the hemisphere shaped volume and the secondsource.
 21. The EUV light source of claim 20, wherein the pulse of lightis 150 ps or less in duration.
 22. An extreme ultraviolet (EUV) lightsource comprising: a first source that produces a pulse of light; asecond source that produces an amplified light beam; a target materialdelivery system; a chamber coupled to the target material deliverysystem; and a steering system that steers the amplified light beamtoward a target location in the chamber that receives a target materialdroplet from the target material delivery system, the target materialdroplet comprising a material that emits EUV light after being convertedto plasma, wherein the target material droplet forms a target whenstruck by the pulse of light, the target comprising a hemisphere shapedvolume having pieces of the target material throughout the volume, and aplane surface positioned between the hemisphere shaped volume and thesecond source, wherein the pulse of light is 150 ps or less in duration.23. The EUV light source of claim 22, wherein the pulse of light has awavelength of 1 micron (μm).
 24. The EUV light source of claim 22,wherein the hemisphere shaped volume comprises a rounded portion havinga circumference and a planar portion extending throughout a regiondefined by the circumference.
 25. A method comprising: releasing aninitial target material toward a target location, the target materialcomprising a material that emits extreme ultraviolet (EUV) light whenconverted to plasma; directing a first amplified light beam toward theinitial target material, the first amplified light beam having an energysufficient to form a collection of pieces of target material from theinitial target material, each of the pieces being smaller than theinitial target material and being spatially distributed throughout ahemisphere shaped volume; and directing a second amplified light beamtoward the collection of pieces to convert the pieces of target materialto plasma that emits EUV light, wherein the first amplified light beamcomprises a pulse of light having a duration of 150 picoseconds (ps) orless and a wavelength of 1 micron (μm).
 26. The method of claim 25,wherein the second amplified light beam occurs 200-1600 ns after thefirst amplified light beam.
 27. The method of claim 25, wherein thehemisphere shaped volume comprises a rounded portion having acircumference and a planar portion extending throughout a region definedby the circumference.
 28. A method comprising: releasing an initialtarget material toward a target location, the target material comprisinga material that emits extreme ultraviolet (EUV) light when converted toplasma; directing a first amplified light beam toward the initial targetmaterial, the first amplified light beam having an energy sufficient toform a collection of pieces of target material from the initial targetmaterial, each of the pieces being smaller than the initial targetmaterial and being spatially distributed throughout a hemisphere shapedvolume; and directing a second amplified light beam toward thecollection of pieces to convert the pieces of target material to plasmathat emits EUV light, wherein a density of the pieces of the targetmaterial in the hemisphere shaped volume increases along a directionthat is parallel to a direction of propagation of the amplified lightbeam.
 29. The method of claim 28, wherein the hemisphere shaped volumecomprises a rounded portion having a circumference and a planar portionextending throughout a region defined by the circumference.